Method of zone refining for impurities having segregation coefficients greater than unity



Aug 15 1961 R. E. JOHN oN 2996374 METHOD 0F zoNE REEINING FORIMPURSITIES HAVING sEGREGATIoN' COEEEICIENTS GREATER THAN UNITY FiledNov. 13, 1958 /MPUP/TY C' ONCE/171647" /O/V BY MEM ATTORNEYS UnitedStates Filed Nov. '13, 1958, Ser. No. '773,737 4 Claims. (Cl. 75-'63)The present invention relates to a method for purifying semiconductormaterials and more particularly relates to` a zone refining 4techniqueespecially applicable for processing impurities having segregationcoefficients greater than unity.

yOne of the methods currently used for purifying semiconductor materialsis a technique known as zone refining. Such a technique is described,for example, by W. G. Pfann, Principles of Zone Melting, Journal ofMetals, July 1952, pages 747-753, and by the same author Segregation ofTwo Solutes, with Particular Reference to Semiconductors, Journal ofMetals, August 1952, pages 861-865. The method relies on the fact thatmany Semiconductor carrier impurities are more soluble in liquidsemiconductor material than in solid material, thus enablingpurification to result from progressive melting and solidification in abar of semiconductor material. In carrying out a zone melting operation,a bar of semiconductor material is subjected to local heating, whichresults in the formation of a small molten zone. The zone is moved alongthe bar either by moving the heating source along the bar while holdingthe bar stationary or by moving the bar relative to a stationary heater.

IIn describing zone refining techniques, it is convenient to refer to asegregation coefiicient defined as the ratio of the concentration of theimpurities in the solid phase to the concentration of impurities in theliquid phase. Thus, impurities which have a segregation coefficientsmaller than unity are more soluble in liquid than in solid and tend toremain in the liquid phase. As a molten zone is caused to traverse thelength of a semiconductor bar, these impurities are carried along by themolten zone, and they tend to segregate to the last portion of the barto solidify. The end of the bar solidifying first is left relativelyfree from imptuities.

The above-described zone refining technique, however, does not work aswell when dealing with substances having segregation coefficientsgreater than unity. Such substances prefer the solid phase to the liquidphase, and thus when a molten zone pass is made in a bar ofsemiconductor material, impurities with segregation coefficients greaterthan unity move slightly toward the first portion of the bar tosolidify. The degree of segregation is slight, however.

A process has been tried in which a bar of semiconductor material isuniformly heated to the molten state, and then is progressivelysolidified by subjecting one end to cooling means and causing the moltenmaterial to solidify unidirectionally from one end of the bar to theother end. Such a process results in some degree of segregation, sincethe impurities with segregation coefficients greater than unity tend toaccumulate in the first portion of the bar to solidify. However, thisprocess has the drawback that there is no appreciable improvement insegregation upon repetition of the process.

Therefore, it is a principal object of the present invenatent ice tionto provide an improved method of zone refining which will enablesemiconductor materials with impurities having segregation coefiicientsgreater than unity dispersed therein to be purified to a greater extentthan can be accomplished by present methods.

It is a further object of the present invention to provide a zonerefining technique for impurities having segregation coefiicientsgreater than unity which may be repeated `as many times as is necessaryto obtain a desired degree of segregation and which technique givesimprovements in segregation when repeated.

Other objects and advantages of the present invention will becomereadily apparent from the following detailed description of a preferredembodiment of the invention when taken in conjunction with the drawingin which:

FIGURE 1 is a graph showing the impurity concentration profile of a barof semiconductor material when subjected :to the various stepsconstituting the purifying technique of the present invention; `andFIGURE 2 is a graph showing zone width versus zone position.

Referring to FIGURE 1, the impurity concentration as a function of barlength for an untreated bar is shown by the heavy solid line on thegraph to be uniform throughout the length of the bar. For illustrativepurposes, the impurity concentration is shown to have a relativemagnitude of unity. :The impurity chosen for this and following exampleshad a segregation coefficient of three.

In the zone melting technique of the present invention the bar ofsemiconductor material is first subjected to uniform heating until allthe material is in the molten state. The bar is then subjected to asolidifying operation. The solidifying is accomplished by applyingcooling means to one end (designated as the starting end) of thesemiconductor bar and by this means, the molten material is solidifiedunidirectionally. As -is shown by the dashed line on the graph, themelting and progressive cooling technique results in some degree ofsegregation for irnpurities having segregation coefficients greater thanunity. The impurities tend to concentrate in the starting end, which isthe rst portion of the bar to solidify. Asis shown in the drawing, atthe starting end the impurities are present in a concentration ofrelative magnitude three, while at the nal end, the impurityconcentration has decreased to zero.

Next, a molten zone is caused to traverse the length of the bar,starting from the same end lthat the cooling operation was started. Thewidth of the molten zone will be adjusted in accordance with theimpurity profile of the bar. After a preliminary melt-in of a narrowzone at the starting end of the bar, the zone is adjusted in width tomaintain the maximum concentration of impurity in the molten zone. Thisinsures Ithat a maximum amount of impurity is deposited with thefreezing interface. For illustrative purposes an example will be givenusing an impurity whose segregation coefficient is three. lf `the bar isfrozen in one direction as described previously, the relative magnitudeof impurity will be three at the first frozen end and will decreasealong the bar as indicated by the dashed curve on the graph.

lf now ya molten zone of variable width with the following program ispassed along the bar, the impurity will tend to segregate significantlyin the first frozen end. The units in the following table are infractional length of bar, measured from first-frozen end.

Position of Position of Width of Freezing Face Melting Face Molten Zone0. 0. 05 0.05 (melt-in zone) 0. 05 0. 29 0. 24 0. 0. 41 0. 31 0. 0. 490. 34 0. O. 56 0. 36 0. 0. 62 0. 37 0. 3() 0. 67 0. 37 O. 0, 71 0. 36 0.0. 75 0. 35 0. 0. 78 U. 33 0. 5() 0. 82 0. 32 0. 0. 84 0. 29 0. 0. 87 O.27 0. D. 89 0. 24 0. (l. 92 0. 22 0. 0. 93 0. 18 0. 0. 95 O. 15 0. 0. 970. 12 0. 0.98 0. 08 D. 0. 99 0. 04 l. 00 1. 00 0. 00

it is not claimed that this is the ultimate refinement in zone width;rather the technique is constrained by experimental limitations of thepresent apparatus. Thus, if the melt-in zone is made narrower, theultimate purification would be better. Further, if finer control on thewidth of the zone were possible, again puriiication would be better. Thefigures quoted represent -a reasonable set of values for the presentstate (or possible refinements) of the zone reiining process. n l Forthe conditions as in the above program, the impurity concentration wouldapproach a value of relative magnitude 9 in the first frozen end of thebar and would have a distribution along the bar represented by thedotdash curve in the ligure.

=If a second molten zone pass is now made, still greater segregationresults. The width of the second molten zone must be programmed inaccordance with the impurity profile of bar, hence a new program must becalculated for each successive pass. In the second pass, the impuritylevel in the iirst frozen end should approach 27 for the example; inactual practice, it will be closer to 22 due to practical limitations ofthe experimental apparatus. The impurity proiile after the second passis represented on the graph by the dotted line.

Still greater segregation can be obtained by employing the followingprocedure. A bar of semiconductor material is first puriied according tothe above-mentioned technique. Then a portion of the starting end of thebar is cropped or cut oi, after which the entire process is repeated ormore desirably only the step of subjecting the bar to a molten zone passas previously described. In every case the molten zone pass is startedfrom the same end and the Width of the molten zone is programmed tomaintain a maximum concentration of impurity in the molten zone. Theover-all operation of employing the basic zone melting technique of thepresent invention, cutting off a portion of the starting end of the barof semiconductor material, and then repeating the molten zone passtechnique several times can produce bars having a desirable degree ofsegregation. Y

Although the present invention has been shown and described withreference to a particular embodiment, nevertheless various changes andmodifications obvious to one skilled in the art are within the spirit,scope and contemplation of the present invention.

What is claimed is:

1. A method of zone refining for impurities having segregationcoeiiicients greater than unity comprising the steps of heating a bar ofsemiconductor material to the molten state, solidifying said barprogressively from one end to the other, causing a molten zone totraverse the length of said solid bar starting from the said one end,and non-linearly increasing the yvolume of said molten zone until thefreezing face is positioned at about 25% of the bar length andthereafter decreasing the volume of the molten zone to maintain amaximum concentration of impurities in said molten zone.

2. A method as deiined in claim l wherein the passage of said moltenzone along the length of said bar is repeated as many times as isnecessary to obtain a given degree of segregation.

3. A method of zone refining for impurities having segregationcoetlicients greater than uni-ty comprising the steps of heating a barof semiconductor material to the molten state, solidifying said barprogressively rom one end to the other, causing a molten zone totraverse the length of said bar starting from said one end, non-linearlyincreasing the volume of said molten zone until the freezing face ispositioned at about 25% of the bar length and thereafter decreasing thevolume of the molten zone to maintain a maximum concentration ofimpurities in said molten zone, and cutting oli a portion of said bar atsaid one end and subjecting the remaining portion of said bar to asecond pass of a molten zone.

4. A method of zone refining for impurities having segregationcoefficients greater than unity comprising the steps of heating a bar ofsemiconductor material to the molten state, progressively solidifyingsaid bar from one end to the other, causing a molten zone to traversethe length of said bar starting from said one end, non-linearlyincreasing the volume of said molten zone until the freezing face ispositioned about 25% of the bar length and thereafter decreasing thevolume of the molten zone to maintain a maximum concentration ofimpurities in said molten zone, repeating the step of causing the moltenzone to traverse the length of said bar starting from said one end andthe step of non-linearly increasing the volume of said molten zone untilthe freezing face is positioned at about 25% of the bar length andthereafter decreasing the volume of the molten zone to maintain amaximum concentration of impurities in said molten zone, cutting olf laportion of said bar at said one end and subjecting the remaining portionof said bar to another pass of said molten zone.

OTHER REFERENCES Pfann, William G.: Zone Melting, New York: John Wiley &Sons, March, 1958, pp. 8 15.

1. A METHOD OF ZONE REFINING FOR IMPURITIES HAVING SEGREGATIONCOEFFICIENTS GREATER THAN UNITY COMPRISING THE STEPS OF HEATING A BAR OFSEMICONDUCTOR MATERIAL TO THE MOLTEN STATE, SOLIDIFYING SAID BARPROGRESSIVELY FROM ONE END TO THE OTHER, CAUSING A MOLTEN ZONE TOTRAVERSE THE LENGTH OF SAID SOLID BAR STARTING FROM THE SAID ONE END,AND NON-LINEARLY INCREASING THE VOLUME OF SAID MOLTEN ZONE UNTIL THEFREEZING FACE IS POSITIONED AT ABOUT 25% OF THE BAR LENGTH ANDTHEREAFTER DECREASING THE VOLUME OF THE MOLTEN ZONE TO MAINTAIN AMAXIMUM CONCENTRATION OF IMPURITIES IN SAID MOLTEN ZONE.